This invention relates to an apparatus for removing nitrogen oxides (NO.sub.x) from a combustion flue gas, and particularly to an apparatus for removing NO.sub.x by adding a reducing agent to the combustion flue gas, thereby decomposing NO.sub.x by gas phase reduction in the absence of a catalyst, and by providing a temperature-controllable bed of catalyst downstream of the stage in said apparatus where reducing agent is added to obtain a high percent NO.sub.x removal.
Combustion flue gas from various industrial combustion equipments using a fossil fuel, such as boilers and gas turbines, contains nitrogen oxides formed in the combustion process. Nitrogen oxides themselves are toxic, and are materials causing photo-chemical smogs. Accordingly a prompt development of techniques for abating NO.sub.x in the combustion flue gas has been keenly desired.
NO.sub.x is formed in a high temperature zone of a flame in the combustion equipment, and the amount of NO.sub.x formed is increased at a higher temperature and by combustion in richer oxygen.
Nowadays, the techniques of abating NO.sub.x in boilers and gas turbines are classified into two main groups, that is, the group of techniques of combustion with low NO.sub.x content and the group of techniques of removal of NO.sub.x from flue gas. The former group is based on a combustion at a low temperature in a low oxygen content, and typical processes of this type are a two-stage combustion process, a flue gas recycle process and a diluted combustion process. The latter group of techniques for removing NO.sub.x from flue gas includes a gaseous phase reduction process comprising adding hydrocarbons, hydrogen, carbon monoxide and ammonia to a combustion flue gas at a relatively high temperature to decompose NO.sub.x in a gas phase reduction, and a catalytic reduction process comprising adding ammonia, etc. to a combustion flue gas at a relatively low temperature, for example, 250.degree. to 450.degree. C. and decomposing NO.sub.x in the presence of a catalyst by gas phase reduction, as disclosed, for example, in U.S. Pat. No. 3,900,554.
The gas phase reduction process is new, as compared with the catalytic reduction process, and belongs to a new technical field involving various problems, but seems to be capable of being greatly advanced by the future technical development. When hydrocarbons, hydrogen and carbon monoxide or the like are used as the reducing agents in the gas phase reduction process, these reducing agents react not only with NO.sub.x, but also residual oxygen in the combustion flue gas, and thus the consumption of the reducing agents is increased, rendering the process uneconomical. If ammonia is used as a reducing agent on the other hand, ammonia selectively reacts with NO.sub.x and thus the consumption of the reducing agent is small, and also the percent NO.sub.x removal is higher than that of the former process. Thus, the selective reduction process using ammonia is especially remarkable in the gas phase reduction processes.
However, according to the conventional gas phase reduction process using ammonia, the reacting temperature necessary for the NO.sub.x reduction is high, for example, at least 800.degree. C., and when an application thereof to an existing combustion apparatus such as a boiler or gas turbine is taken into account, there are various problems due to such high temperature conditions e.g. a residence time of a high temperature gas is short, a uniform satisfactory diffusion of ammonia into the combustion flue gas is hardly attainable, etc. Especially in the case of a gas turbine, the temperature zone for removing NO.sub.x is within the turbine stage, and thus the application of such process is actually impossible. To solve these problems, a gas phase reduction process applicable to a low temperature range is now in development. As one of such processes, a gas phase reduction process comprising adding ammonia and hydrogen peroxide to a flue gas is available, and its principle of removing NO.sub.x is to decompose ammonia to active chemical species, for example, amino radical, imino radical, etc. in advance by reaction between ammonia and hydrogen peroxide and then to decompose NO.sub.x by reaction of these active chemical species by reduction. The effective temperature for NO.sub.x removal reaction can be lowered to such a low temperature range as about 400.degree. C. in said process. In this process the necessary amount of ammonia for effectively decomposing NO.sub.x in the flue gas by reduction is in about 0.3-about 10 in terms of a molar ratio of ammonia to NO.sub.x (NH.sub.3 /NO.sub.x), and preferably about 0.5-about 3 in view of the economy and prevention of unreacted ammonia discharge. The amount of hydrogen peroxide to be added thereto is that necessary for decomposing ammonia, and is about 0.3-about 1 in terms of molar ratio of hydrogen peroxide to NO.sub.x (H.sub.2 O.sub.2 /NO.sub.x).
However, in this gas phase reduction process using ammonia and hydrogen peroxide, there are such disadvantages that, since oxidation of NO by hydrogen peroxide takes place, the percent NO.sub.x removal is somewhat lower than the ammonia reduction process, and when ammonia is added in excess to increase the percent NO.sub.x removal, unreacted ammonia is discharged. That is, it is difficult to obtain a satisfactory percent of NO.sub.x removal in any of the processes for removing NO.sub.x from the flue gas.